Check out
ftp://ds.internic.net/internet-drafts/draft-levinson-cid-01.txt
> In other words, I want to send an encapluated image/gif within a mail message. The
> image is to be referenced from a text/html section. Note that a hyper link (like
> http) is to be avoided.
> A second related question is how to instruct the receiving end to show the
> image only from the text/html section and not from the image/gif section.
I believe the proposal will be to use multipart/related as specified
in draft-ietf-mimesgml-related-03.txt.